Epoxy resins are commercially important materials that are used extensively to make thermosetting products for use in coatings, adhesives, composites, and many other applications. The largest volume of epoxy resins utilized in commerce are those based upon the diglycidyl ether of bisphenol-F (DGEBF), epoxy novolac resins, and those based upon the diglycidyl ether of bisphenol-A (DGEBA). Of these, the bisphenol-A based products are utilized in much larger volumes than the other products.
Bisphenol-A derived epoxy resins are essentially linear polymers available in a wide range of molecular weights, represented generically by the following chemical structure:
where n represents the average number of repeat units in the polymer. The low end of the range of available molecular weight products are made by reaction of bisphenol-A with excess epichlorohydrin, followed by treatment with base. They are referred to by those who work in the industry as liquid epoxy resin, DGEBA, or BADGE, even though most of the commercial products are not pure DGEBA but often have a value of n of about 0.15 or slightly higher. Higher molecular weight epoxy resins (greater than about 400 Daltons) are commercially prepared by the so-called “advancement process” which is the, reaction of excess DGEBA with bisphenol-A, where the ratio of DGEBA to bisphenol-A is used to control the final average molecular weight.
Because high molecular weight epoxy resins are prepared by the reaction of DGEBA and bisphenol-A, such resins prepared using current commercial processes have relatively high levels of residual bisphenol-A and DGEBA in the final products. Unfortunately, these compounds are of concern with regard to their human health effects and pseudo-estrogenic activity. This is particularly true in the industry for coatings for food and beverage can interiors, where epoxy resins are currently utilized in large volumes for coatings that are crosslinked with amino resins and other OH-reactive crosslinking agents. Thus, there is a strong need to develop coatings with properties similar to those obtained from crosslinked epoxy resins, but without such high levels of residual DGEBA and bisphenol-A, which can be extracted into the contents of the can and thus become a component of the human diet.
Epoxy resins contain epoxide rings at the chain ends, and (with the exception of pure DGEBA) secondary hydroxyl groups spaced along the polymer backbone. Both of these functional groups can be utilized to cure the epoxy resin. For example, multifunctional amines, mercaptans, and carboxylic acids are utilized to crosslink through the epoxide ring. Amino resins such as melamine-formaldehyde and urea-formaldehyde resins, and polyisocyanates are utilized to crosslink through the hydroxyl groups. Finally, resins such as resoles crosslink through both the hydroxyl and epoxide functional groups. For most purposes, epoxy resins that are crosslinked via the epoxide end groups have epoxy equivalent weights (EEW) of at most about 800, and frequently far less than this. On the other hand, when crosslinked through hydroxyl groups, higher molecular weight epoxy resins are generally preferred, and very low molecular weight epoxy resins such as pure DGEBA which lack OH groups cannot be utilized at all in such a thermosetting system.
Despite the fact that epoxy resins can be crosslinked with amino resins and the like through the secondary hydroxyl groups on the resin backbone, it is generally found that significantly higher temperatures and/or bake times are required than are necessary with other polyols utilized in coatings, such as acrylic polyols and polyester polyols. It is thought that the relatively hindered environment of the OH groups on the epoxy resin is responsible for this effect. Obviously, this is usually a significant drawback to the utilization of epoxy resins in such coatings, since higher oven temperatures and/or bake times lead to higher production costs.
The cationic or acid-catalyzed polymerization (or homopolymerization) of multifunctional epoxy resins to yield gelled or crosslinked final products is a well-known process of significant commercial importance. Lewis acids are most commonly employed, but appropriate Brønsted acids can also be utilized. For example, C. A. May (Ed.), Epoxy Resins Chemistry and Technology, Marcel Dekker, Inc.: New York, 1988, reports (p. 495) that Lidarik et. al. (Polymer Sci. USSR, 1984, 5, 859) polymerized glycidyl ethers with complexes of antimony pentachloride, boron trifluoride, and perchloric acid. Additional examples are reported in May. In addition, the photoinitiated cationic polymerization of epoxy resins is well-known, and also of commercial importance. As reviewed in May (pp. 496–498), cationic photoinitiators are materials that upon photolysis generate strong Brønsted acids, which serve as the true catalyst for the epoxide polymerization.
Lidarik et. al. (ibid.) prepared gelled products by the acid catalyzed polymerization of epoxy resins in tetrahydrofuran (THF) solutions, and obtained evidence that THF-derived products were incorporated into the network.
The copolymerization of water with monofunctional epoxide compounds has been known for some time. For example, R. W. Lenz, Organic Chemistry of Synthetic High Polymers, Interscience Publishers: New York, 1967, pp. 531–546, reviews the ring-opening polymerization of cyclic ethers including epoxides, and notes that C. Matignon, et.al. (Bull. Soc. Chim., 1, 1308 (1934)) studied the effect of water content on the oligomer distributions obtained from the acid-catalyzed hydration of ethylene oxide.
U.S. Pat. No. 6,331,583 B1 discloses compositions of emulsified polymeric polyols prepared by a method comprising the acid catalyzed, non-reversible polymerization of lower molecular weight epoxy resins in an aqueous emulsified state. Coating compositions are prepared from the emulsified polymeric polyols crosslinked with various crosslinking agents.
U.S. application Ser. No. 10/062,924, filed 31 Jan. 2002 discloses the preparation of polymeric polyols by the acid-catalyzed solution copolymerization of epoxy resins and water.
U.S. Pat. No. 2,872,427 discloses oil-in-water emulsions of polyepoxide resins and their heat cure with various curing agents, including acid acting curing agents.